The present invention relates to field of electrical defibrillation, including cardioversion, and more particularly to the structure for an electrode used in implantable defibrillation systems. The term "defibrillation", as used herein, includes cardioversion which is another technique involving relatively high energy delivery, as compared to pacing, as well as other aspects of defibrillation therapy such as the monitoring of cardiac electrical activity (sensing) when not delivering high energy impulses.
Defibrillation is a technique employed to counter arrhythmic heart conditions including some tachycardias, flutter and fibrillation in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing. One defibrillation approach involves placing electrically conductive paddle electrodes against the chest of the patient. During cardiac surgery, such paddles can be placed directly against the heart to apply the necessary electrical energy.
More recent defibrillation systems include body implantable electrodes. Such electrodes can be in the form of patches applied directly to epicardial tissue, or at the distal end regions of intravascular catheters, inserted into a selected cardiac chamber. U.S. Pat. No. 4,603,705 (Speicher et al), for example, discloses an intravascular catheter with multiple electrodes, employed either alone or in combination with an epicardial patch electrode. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. No. 4,567,900 (Moore).
Epicardial electrodes are considered the most efficient, in the sense that less energy is required for defibrillation as compared to either chest contact paddles or intravascular catheter electrodes. However epicardial electrode implantation is highly invasive, major surgery, since it is necessary to enter the chest cavity, which typically involves spreading of adjacent ribs or splitting of the sternum. This procedure presents a risk of infection. Further, implantation and attachment place physical constraints upon the nature of electrode. These electrodes must be either quite small, or extremely compliant and resistant to fatigue, as they maintain conformal fit to the contracting heart.
Generally, larger defibrillation electrodes are considered more desirable, since they reduce the impedance at or near the electrode. Sensing artifacts also are reduced for larger electrodes. However, larger electrodes are difficult to attach to the epicardium, as they must conform to the heart during the contractions associated with normal cardiac activity. Subcutaneous electrodes are more easily implanted, at less risk to the patient. In a defibrillation electrode or any other implanted device, however, increasing the size generally increases discomfort and surgical risk to the patient.
Increasing the size of a defibrillation electrode affects its electrical performance. Conventional electrodes are subject to "edge effects" arising from the non-uniform distribution of electrical energy when the electrode receives the pulse. In particular, current densities are greater at the edges of the electrode than at interior regions of the electrode. An attempt to counter the edge effect is disclosed in U.S. Pat. No. 4,291,707 (Hellman et al). A series of circular openings, through an insulative layer framing a conductive screen, are said to substantially eliminate the edge effect by the additional exposure of the screen. Another problem encountered in larger electrodes is the resistance across the length (largest linear dimension) of the electrode, leading to unwanted voltage gradients across the electrode which can degrade electrode performance.
Therefore, it is an object of the present invention to provide an implantable defibrillation electrode with a large effective surface area to lower the impedance at or near the electrode, without causing undue patient discomfort.
Another object is to provide a defibrillation electrode that has a large effective area, yet is easier to implant and readily conforms to the contours of its implant location.
A further object is to provide a defibrillation electrode structure enabling a relatively large size while reducing the non-uniform field distribution associated with conventional electrodes.
Yet another object is to provide defibrillation electrodes of sufficient size and effectiveness to enable transthoracic delivery of defibrillation pulses, with an implanted system.